1. Field of the Invention
The present invention relates to an apparatus for machining a sheet with a die and a punch.
2. Description of the Related Art
Various dies or machining apparatus have generally been used to punch holes or openings in workpieces. For example, there is a prior art progressive die assembly having dies and punches movable toward and away from each other for successively punching holes in a workpiece that is being fed through the dies and punches. There is also a compound blank-and-punch die assembly for blanking and punching a workpiece at once with a die which has holes according to a machining pattern for the workpiece, and as many punches as the number of those holes in the die.
In the prior art progressive die assembly, since only the workpiece is moved, external forces are applied to move the workpiece. The workpiece is normally spaced from the dies so that it can be fed successively through the dies and punches. Therefore, the workpiece is subject to external forces when it is punched by the punches. If the workpiece comprises a thin and/or soft sheet, then the sheet tends to be strained by the external forces applied to move the sheet and also the external forces applied to punch the sheet. When the sheet is strained, it reduces the dimensional accuracy of holes punched therein, particularly if many small holes are punched in the sheet, and it cannot be fed at a highly accurate rate through the dies and punches.
The compound blank-and-punch die assembly has as many punches as the number of holes in the die according to the machining pattern for the workpiece. If the machining pattern has many small holes, then the compound blank-and-punch die assembly is required to have as many punches of small diameters as the number of the holes in the die. It is relatively expensive to manufacture such punches of small diameters, and such punches of small diameters are liable to be damaged in use. As a result, the compound blank-and-punch die assembly with many punches of small diameters needs a considerably complex maintenance process.